Pulmonary arterial hypertension (PAH) is a life-threatening disorder of the pulmonary circulation associated with loss and impaired regeneration of microvessels. Pericytes are highly specialized cells that interact with endothelial cells to promote vessel survival but little is known about their role in repair and regeneration of pulmonary microvessels. One possibility is that failure of pericytes to associate with endothelial cells could contribute to small vessel loss in PAH by reducing regeneration and viability of pulmonary microvessels. Using an in vitro co-culture model, we found that pulmonary microvascular endothelial cells (PMVECs) purified from PAH lungs are unable to recruit healthy lung pericytes; likewise, PAH lung pericytes fail to associate with healthy PMVECs, suggesting an inherent defect in mechanisms responsible for endothelial-pericyte interactions. We have discovered that inability of PAH PMVECs to recruit pericytes is associated with reduced production and release of the Wnt/PCP ligand Wnt5a, since supplementation with recombinant Wnt5a improves pericyte recruitment and vessel formation. Interestingly, we found that PAH pericytes are resistant to Wnt5a stimulation, suggesting a possible defect in Wnt/PCP receptor function. Genetic screening led us to identify ROR2 as a candidate receptor required for Wnt/PCP activation in response to Wnt5a and its loss prevents pericyte recruitment to healthy microvessels. Therefore, we hypothesize that establishment of pulmonary endothelial-pericyte interaction is dependent on PMVEC production of Wnt5a, which attracts pericytes via ROR2 dependent activation of the Wnt/PCP pathway. Through a combination of novel transgenic animal models, molecular biology and gene editing, we will elucidate for the first time how Wnt5a and ROR2 orchestrate communication between PMVECs and pericytes (Aims 1 and 2). In addition, we will also pursue exciting preliminary data that links reduced Wnt5a production in PAH PMVECs to loss of bone morphogenetic protein (BMP) signaling, a key signaling pathway whose dysregulation is linked to PAH (Aim 3). Finally, we will demonstrate that PMVEC exosomes are required to deliver Wnt5a to pericytes and apply a novel technology to isolate and engineer exosomes for in vivo Wnt5a delivery (Aim 4). Understanding how Wnt5a and ROR2 orchestrate endothelial-pericyte interactions can provide insight into the mechanism behind progressive small vessel loss in PAH and open new therapeutic opportunities to promote regeneration of lost vessels, prevent progression and improve clinical outcomes for patients afflicted with this devastating disease. ! Pericyte EC Proposed model. Pulmonary endothelial cells (ECs) produce and release Wnt5a in exosomes, which drives pericyte recruitment by triggering ROR2 phosphorylation (P) and Wnt/PCP activation (top). Loss of BMPR2 in PAH ECs results in reduced Wnt5a production and release with consequent reduced Wnt/PCP activation and failure to establish endothelial-pericyte interactions (bottom).